We have developed a new MR imaging modality, Diffusion Tensor MRI (DT-MRI), to measure this diffusion tensor in vivo, noninvasively. DT-MRI consists of (a) relating an effective diffusion tensor to the measured MR spin echo, (b) estimating an effective diffusion tensor, D, in each voxel from a set of diffusion-weighted MR images (DW-MRIs) and (c) calculating and displaying information derived from D. New microstructural features we are able to glean with DT-MRI include the local fiber-tract orientation, the mean-squared distance water molecules diffuse along any direction, the orientationally averaged mean diffusivity (or Trace), and other scalar invariant quantities derived from D which are independent of the laboratory coordinate system. Recently, we have proposed several new measures of the degree of diffusion anisotropy and the degree of fiber-tract coherence. These scalar parameters behave like quantitative histological or physiological stains, yet they are produced without the use of any exogenous contrast agents or dyes.Collaborations with Richard Shrager (CIT) have resulted in a mathematical demonstration that at least seven DWIs are necessary to characterize water diffusion in tissues adequately. Work with Carlo Pierpaoli has resulted in the development of a simplified method to perform DT-MRI with only seven diffusion weighted images (DWIs). Work with Sinisa Pajevic (CIT) and Akram Aldroubi (Vanderbilt University) has resulted in the development of a general mathematical framework for representing and analyzing diffusion tensor field data. Collectively, these developments are making possible the wide-spread use of this new methodology from bench to bedside.We are continuing to apply DT-MRI to problems in basic research and in clinical medicine. The bulk or orientationally-averaged diffusivity has been the most successful imaging parameter proposed to date to visualize a stroke in progress. DT-MRI has also been shown to be effective in identifying Wallerian degeneration as well as organized gliosis that often result in chronic stroke. Preliminary studies have also shown DT-MRI to be useful in following developmental changes in the cortical gray and white matter of living post-natal cat brain. Presently, new MRI stains are being developed for use in assessing the degree of tissue organization, as well as fiber tract continuity and connectivity. - MRI, diffusion, tensor, anisotropy, fiber connectivity, - Human Subjects